With the advent of smart phones, the amount of mobile traffic has increased explosively, and communication services using the unlicensed band that is not licensed for a particular communication operator, such as Wireless Local Area Network (WLAN) or Bluetooth, have been provided. The WLAN network that can be built at a low cost has attracted attention as a major solution for the cellular operators that can hardly deal with the traffic only with the cellular network, because the WLAN network uses the unlicensed band.
In other words, most cellular communication operators may build the WLAN network by themselves or may enter into partnership with the existing WLAN operator, thereby causing their own subscribers to receive communication services through a WLAN Access Point (AP) if the cellular communication operators can hardly accommodate the traffic of the subscribers only with the cellular evolved Node B (eNB) (also known as a cellular base station) like in the area where there is a lot of floating population.
The WLAN network that the communication operator has installed for traffic offloading is a network that originally has different characteristics from those of the cellular network. Therefore, the WLAN network may be insufficient to provide the mobility (handoff) or security that the communication operator can provide by installing the cellular eNB for the mobile communication subscribers. In addition to the basic method for installing the WLAN AP, the mobile communication operators have studied ways to provide mobile communication services to subscribers by directly using the cellular radio technology in the unlicensed band that is not licensed for the communication operators.
For reference, the term ‘unlicensed band’ as used herein may refer to a communication frequency band that is not licensed for a particular communication operator, and the unlicensed band is a shared band that is open to the public. The unlicensed band may be typically used as a band for industrial, science and medical communication. In addition to the WLAN (or Wireless Fidelity (Wi-Fi)), Bluetooth is the typical communication service that uses the unlicensed band.
For reference, the WLAN technology will be described below. The WLAN is used around the bands of 2.4 GHz and 5 GHz based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. However, the trend is that the latest WLAN standards are designed around 5 GHz having a broader band, getting out of the band of 2.4 GHz that is currently overcrowding due to the narrowness of the whole band.
On the other hand, as for the WLAN channel, a channel of 20 MHz is defined as a basic channel, and as for the broadband channel, the band is extended in such a manner that channels of 40 MHz, 80 MHz, 160 MHz and the like are configured based on the basic channel of 20 MHz. In the IEEE 802.11n standard that uses a bandwidth of 40 MHz through channel bonding to maintain the backward compatibility with the IEEE 802.11a/b/g standard that was supporting only the bandwidth of up to 20 MHz in the past, a channel of 40 MHz is divided into a primary channel of a bandwidth of 20 MHz and a secondary channel of the remaining bandwidth of 20 MHz, and then maintained.
The IEEE 802.11ac standard supporting a broadband of up to 160 MHz (Optional 160 MHz, Mandatory 80 MHz) may also divide the whole band into a primary channel and a secondary channel and grant different roles thereto in a similar manner. For example, when a broadband of 80 MHz is used, a primary channel of 20 MHz is configured and Carrier Sense Multiple Access (CSMA)/Collision Avoidance (CA) is operated in the primary channel, and the remaining 60 MHz is operated as a secondary channel. However, among the 60 MHz, 20 MHz contiguous to the primary channel is referred to as a ‘secondary-20 channel’, and the remaining 40 MHz is referred to as a ‘secondary-40 channel’. Even in the case of a broadband of 160 MHz (contiguous 160 or noncontiguous 80+80), the broadband is extended in the same manner and a ‘secondary-80 channel’ is defined and used.
FIG. 1 illustrates a relationship among a primary channel and a plurality of secondary channels in an IEEE 802.11 ac WLAN that is operated with 160 MHz according to the related art.
Referring to FIG. 1, the relationship among the primary channel and the plurality of secondary channels illustrates a primary channel 101, a secondary-20 channel 103, a secondary-40 channel 105 and a secondary-80 channel 107 in an IEEE 802.11ac WLAN that is operated with 160 MHz according to the related art is illustrated.
FIG. 2 illustrates a relationship among a primary channel and a plurality of secondary channels in an IEEE 802.11ac WLAN that is operated with 80+80 MHz according to the related art.
Referring to FIG. 2, the relationship among the primary channel and the plurality of secondary channels illustrates a primary channel 201, a secondary-20 channel 203, a secondary-40 channel 205 and a secondary-80 channel 207 in an IEEE 802.11ac WLAN that is operated with 80+80 MHz according to the related art is illustrated.
On the other hand, in IEEE 802.11n/ac or the WLAN standard that uses a channel bandwidth of 40 MHz or more, a primary channel and a secondary channel are used for different purposes. In the case of the IEEE 802.11n standard, a User Equipment (UE) (or an AP) desiring to transmit a Physical-layer Protocol Data Unit (PPDU) of 40 MHz performs channel contention (CSMA/CA) through the primary channel. In the channel contention process, just before a backoff counter expires, the UE (also known as a terminal or a mobile station (MS)) performs Clear Channel Assessment (CCA) to determine whether a secondary channel is busy or idle, for a certain period of time. The certain period of time is determined as Distributed InterFrame Space (DIFS) in the case of a band of 2.4 GHz, and as Point InterFrame Space (PIFS) in the case of a band of 5 GHz. As a result of the CCA, if the secondary channel is in an idle state, the UE may transmit a PPDU of 40 MHz using a primary channel and a secondary channel, but if the secondary channel is busy, the UE should transmit a PPDU of 20 MHz using only the primary channel, or restart the channel contention.
The IEEE 802.11ac standard also defines a similar channel access rule. For example, a UE desiring to transmit a PPDU of 160 MHz first performs channel contention in a primary channel depending on the standard, and just before a backoff counter expires, the UE performs CCA to determine whether a secondary-20 channel, a secondary-40 channel and a secondary-80 channel are in an idle state, during a PIFS time period. If the primary channel, the secondary-20 channel, the secondary-40 channel and the secondary-80 channel are all in the idle state, the UE may transmit the PPDU of 160 MHz. If the primary channel, the secondary-20 channel and the secondary-40 channel are all in the idle state, the UE may transmit a PPDU of 80 MHz. If the primary channel and the secondary-20 channel are all in the idle state, the UE may transmit a PPDU of 40 MHz. If only the primary channel is in the idle state, the UE may transmit a PPDU of 20 MHz. On the other hand, if the primary channel is busy, the UE should restart the channel contention by starting a backoff procedure.
Therefore, a need exists for a method and an apparatus for performing cellular communication using an unlicensed band in a cellular communication system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.